This invention relates to a method for the precipitation of nickel from its aqueous solution as a metal powder using hydrogen. The aqueous solution containing nickel compounds is neutralised first with an earth alkali or alkali compound so the nickel precipitates as nickel hydroxide or as an alkaline salt, after which reduction is performed continuously in the presence of a catalyst in ion form in atmospheric or close to atmospheric conditions.
According to the prior art, the hydrometallurgical production of nickel powder generally takes place with hydrogen gas. The most commonly used method is reduction from an ammoniacal sulphate solution, into which nickel has been dissolved as an ammine complex. These kinds of methods have been described by Sherritt Gordon Mines and Amax. In these methods the neutralising agent, ammonia, is added to the nickel sulphate solution, after which hydrogen is conducted into the solution and reduction takes place. This is described in the following reaction equation: 
The above reduction is a heterogeneous reaction, which requires a catalyst to begin. Many substances are used as catalyst, but iron sulphate FeSO4 is widely used, which precipitates as iron hydroxide Fe(OH)2 when it is added to an alkaline solution. It is assumed that iron hydroxide forms active nuclei on top of which the nickel begins to reduce. When reduction progresses further, the nickel powder itself begins to act as a reduction-promoting catalyst and the reaction proceeds autocatalytically.
Ammonia is a good neutralising agent because it and the ammonium sulphate it generates are water-soluble. Ammonium sulphate can also be recovered by evaporation and crystallisation and used as fertiliser or similar raw material. However this is not always profitable. For these situations other neutralising agents, cheaper than ammonia, have been sought and mention of these can be found in the literature. One particularly interesting alternative is of course lime, which is one of the cheapest neutralising agents and offers the possibility of removing the sulphate from the solution as gypsum. Magnesium oxide also has its benefits.
A method is described in GB patent 1,231,572 in which a neutralising agent other than ammonia is used. The method has never been adapted for industrial production and the reason is no doubt the special technology demanded by the severe conditions of the method. The following reactions occur in the said method: 
In the conference publication W. Kunda et al; Proceed. Int. Powder Met. Conf., New York, 1965, (ed. H. H. Hausner, Pergamon, 1966) it is presented on pp. 15-49 that the reduction of nickel hydroxide did not occur at a temperature of 175xc2x0 C. and hydrogen pressure of 350 psi (≈25 bar), not even in the presence of a catalyst. The symposium publication R. Derry, R. G. Whittemore; Int. Symp. on Hydrometallurgy 1973, Chicago, mentions on page 42 that seed crystals are not needed in these kinds of systems, and on page 54 that only a slight reduction of the slurry was possible at a temperature of 170xc2x0 C. even though less than the stoichiometric amount of lime was used, but at a temperature of 200xc2x0 C. reduction proceeded quickly as long as there was not an excess of lime. In these tests the partial pressure of hydrogen was 15-40 bar.
In the method now developed the aqueous solutions of nickel are first treated in the known way so that a nickel compound, such as for instance nickel sulphate in aqueous solution, is neutralised with an earth alkali or alkali compound in order to precipitate the nickel. In this way nickel precipitate is formed, which is either nickel hydroxide or an alkaline salt of nickel, and according to the invention it is possible to reduce the nickel from a slurry of said precipitate in much easier conditions than those described above and even as a continuous process. It is stated in the publications mentioned above that a hydroxide slurry is autocatalysing, even though it demands high pressure and temperature. We have, however, now found that using an external catalyst enables reduction to take place in considerably easier conditions than those described above, i.e. in atmospheric or close to atmospheric conditions. Surprisingly it has also been found that the key factor is not only the catalyst itself, but how the catalyst is fed into the process. An essential feature of the invention is that the catalyst is at least partially in ion form in the solution in the reduction stage of the nickel precipitate slurry and that preferably the catalyst is introduced into the nickel precipitate at the same time as the reducing agent, at least in the early stages of reduction. The essential features of the invention will be made apparent in the attached patent claims.
It has now been found that, for instance, divalent iron in the solution (in ion form) is a strong catalyst for the reduction of nickel hydroxide slurry to such an extent that reduction proceeds quickly at temperatures of even less than 100xc2x0 C. and at atmospheric pressure. Tests carried out have proved that reduction begins at as low as 60xc2x0 C. and is significant at 80xc2x0 C. at an H2- pressure of 0.5 bar. Reduction is carried out preferably at temperatures between 80-130xc2x0 C. and hydrogen partial pressure of 0.5-6 bar. Naturally the method also works at both higher temperatures and higher partial pressures of hydrogen, but in that way the essential benefit of the invention is lostxe2x80x94functioning in atmospheric conditions or at slight overpressure.
In addition to divalent iron, at least partially dissolved divalent chrome, Cr2+, can also be used as catalyst. The method now developed is also easy to adapt for a continuous process, which would lower both investment and operating costs considerably.
In the method according to the present invention, a little less than the stoichiometric amount, 70-98%, preferably 95-98%, of a neutralising agent such as CaO, Ca(OH)2, NaOH, MgO or other suitable alkali or earth alkali compound is added to a nickel sulphate solution in order to precipitate nickel as nickel hydroxide or an alkaline nickel salt. Ammonia can also be used as catalyst, if so desired. As stated above, the advantage of lime is its reasonable price and possibility to remove sulphate as gypsum.
A small amount of FeSO4 in aqueous solution is added to the nickel hydroxide slurry as catalyst so that at least some of the iron is ion form in the solution. Hydrogen acting as reduction gas is added immediately to the solution so that reduction can begin without delay. Hydrogen gas is added until all the nickel has been reduced. However, the invention is not restricted to this procedure, as other methods can be used so long as the procedure ensures the presence of iron ions (or chrome ions) acting as catalyst in the solution when the hydrogen gas is introduced. The method operates according to reaction (2) presented above.